Microfluidic devices are used to perform biological or chemical reactions by manipulating a small amount of fluid. Microfluidic devices may include microfluidic structures arranged within a platform having various shapes such as a chip and a disc. A microfluidic structure may include a chamber which stores a fluid, a channel through which the fluid flows, and a valve which controls fluid flow. The chamber, the channel, and the valve may be disposed in various combinations within a platform. A hydraulic filter is a system for capturing a target material by using a flow of fluids due to a microfluidic structure. A biochip is formed by arranging such microfluidic structures on a chip-type platform so as to perform various assays, including biological reactions, on a small chip.